Refactor node_exporter to support collectors.

A collector is a type matching 'Collector' interface.

The following collectors where added:
- NativeCollector wrapping the original functionality (attributes, load)
- GmondCollector scraping ganglia's gmond (based on gmond_exporter)
- MuninCollector scraping munin (based on munin_exporter)

main.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"flag"
+	"github.com/prometheus/node_exporter/exporter"
+	"log"
+)
+
+var (
+	configFile = flag.String("config", "node_exporter.conf", "config file.")
+	memprofile = flag.String("memprofile", "", "write memory profile to this file")
+)
+
+func main() {
+	flag.Parse()
+
+	exporter, err := exporter.New(*configFile)
+	if err != nil {
+		log.Fatalf("Couldn't instantiate exporter: %s", err)
+	}
+	exporter.Loop()
+}